1. Field of the Invention
The present invention relates to a circuit substrate, and more particularly, to a circuit structure and a circuit substrate for modifying characteristic impedance by using different reference planes.
2. Description of the Related Art
Motherboards are an essential part of computers, controlling signal and data transmission, such as digital signals or analog signals, between devices. Generally speaking, signals transmitted in or between computers are presented by separate-type digital signals, such as signals with a high-voltage or low-voltage level, rather than the continuously varying signals as analog signals. Therefore, when analog signals are inputted from peripheral devices to a processor, an analog-digital transformation process is performed to transfer the analog signals into digital signals to complete signal transmission.
Additionally, in circuit layouts, characteristic impedance for analog signals should be higher than that for digital signals. However, when analog signals and digital signals coexist in the same motherboard, analog signals and digital signals of the same plane share the same reference plane, such as a power plane or a ground plane as a voltage reference for transmitting the analog and digital signals. This method, however, has the following problems. The high characteristic impedance for the digital signals would result in impedance mismatch. In consequence, electrical-magnetic interference (EMI) becomes more serious.
The conventional method for resolving the impedance mismatch is by increasing the line width of the digital signal line or reducing the line width of the analog signal line to modulate the impedance mismatch. However, since the digital signal line and the analog signal line are on the same plane, if the line width of the digital signal line is increased, the original line spaces of the layout would change, and the density of the layout would decline. Additionally, for the line width of the analog line to be reduced, more sophisticated equipment is required for the process, and the manufacturing costs of motherboards will be higher.